The preparation of polyethylenes having specific properties through high-activity Ziegler-Natta catalyst systems has been the object of several publications.
Particularly, Brazilian patent PI BR 8005670 of the Applicant and fully incorporated herein as reference teaches the preparation of a catalyst support based on a high surface area and pore volume alumina that allows ethylene polymerization to reach extremely high molecular weights. The preparation of the alumina used as catalyst support comprises calcining an ammonium dawsonite, (dawsonite is a basic aluminum and ammonium carbonate) at temperatures between 350° C. and 750° C. Said dawsonite is prepared through reaction between ammonium bicarbonate and aluminum sulfate or nitrate, in the presence of ammonium hydroxide followed by precipitation.
The so-obtained alumina has a high pore volume while combining high surface area and high purity. As is known by the experts, for Ziegler-Natta catalysts, features such as specific area, pore dimension and pore volume distribution in alumina are deeply linked to the catalytic activity of these particles. Alumina as described in the cited PI BR 8005302 has pore volume in excess of 1.0 cm3/g and surface area between 200 m2/g and 500 m2/g.
The so-prepared alumina was used by the Applicant in the development of several processes for preparing polyethylene of varying characteristics, including ultra-high molecular weight polyethylene, a polymer the molecular weight Mw of which is higher than 106 and MI21 is zero.
Thus, U.S. Pat. Nos. 6,018,006 and 6,133,188 (corresponding to Brazilian patents PI BR 8703935 and PI BR 8801441) of the Applicant and equally fully incorporated herein by reference teach ethylene polymerization in the presence of a catalyst based on the above-described alumina and between 15% and 85% by weight magnesium chloride previously treated with ethyl benzoate, followed by impregnation with of from 1.3% to 2.0% by weight of titanium tetrachloride. The process also involves a triethyl aluminum (TEA) co-catalyst at a Al/Ti molar ratio of from 40/1 to 100/1. Pressure levels are 3 bar hydrogen and 6 bar ethylene, at temperatures between 80° C. and 90° C., during one hour or more. The molecular weight distribution (Mw) of the obtained polyethylene products is in the range of 3.5 to 6.5% by weight of the total chains having Mw higher than 106 and from 35 to 55% by weight of the total chains having Mw between 105 and 106, with MI21 between 0.32 and 0.10 g/10 minutes.
In spite of the improvement brought about by the above-described technology, and linked to the possibility of varying the molecular weight of polyethylene products by varying the amount of magnesium chloride in the mixed support, it is not possible to find in the above patents the slightest awareness that by varying the ethylene/hydrogen ratio, besides the adjustment of further process conditions, it is possible to obtain new, not yet predicted materials, such as the polyethylene fibers that constitute the object of the present invention.
This is because the ethylene polymerization technology taught in the above patents does not contemplate the ethylene polymerization process conditions in terms of ethylene/hydrogen ratio leading to polyethylene grades endowed with mechanical properties that involve high modulus, while at the same time being extrudable into filaments or fibers, that is, products that can be extruded in conventional extruders, dispensing with the use of solvents as in the gel spinning process used for producing ultra-high molecular weight polyethylene fibers.
On the other hand, the manufacture of naval and offshore cables is of paramount importance in the petroleum industry. Presently used, polyester cables do not float in water, this being a drawback to the user.
In principle, it is possible to shift this manufacture to olefin polymers, such as polyethylene, the density of which is lower than that of water. However, since mechanical properties involve high modulus features, only ultra high molecular weight polymers can be used, this implying in processing such polyethylene through gel spinning processes.
Gel spinning involves dissolving the polymer resin in an organic solvent such as decalin, a toxic and expensive product, and then processing the obtained solution into a fiber, with solvent evaporation/recovery. The overall process is of high cost and requires special measures related to hazards to humans and the environment caused by the solvents used.
The gel spinning process is generally described in U.S. Pat. No. 4,344,908, where it is taught a process for making polymer filaments of high tensile strength and high modulus through stretching of a polymer filament containing a substantial amount, at least 25 wt %, of a solvent for the polymer at a temperature between the polymer swelling and melting points. This technology uses polymer filaments of various kinds, including polyolefins, of high and ultra high molecular weight, without specifying the kind of polyolefin or the preparation of a specific polyolefin.
U.S. Pat. No. 5,256,358 describes a method for preparing filaments from a commercial, ultra-high molecular weight polyethylene through extrusion and stretching. Based on a polyethylene of intrinsic viscosity of at least 3.5 dl/g, filaments are obtained having outer diameter between 0.1 and up to 10 mm and tensile strength of up to 100 kg/mm2. Also described is an extruder having a grooved cylinder having an extrusion orifice of L/D ratio of up to 100, such equipment allowing the extrusion of the polyethylene as threads or filaments. After extrusion, the polymer is drawn or stretched at draw ratios between 1.2/1.0 and 30/1, and the polymer is obtained as filaments. The polyethylene useful for the purposes of said patent is a commercial, ultra-high molecular weight polyethylene having intrinsic viscosity of up to 16.5 dl/g, melt index lower than 0.01 g/10 minutes, melting point 136° C. and bulk density 0.45 g/cm3.
U.S. Pat. No. 5,256,358 describes therefore a non-conventional extrusion equipment as well as a processing method that make possible to process ultra-high molecular weight polyethylene commercial samples into threads or extrudable filaments, having a diameter of up to 10 mm. The obtained filaments are not exactly fibers, that normally have a lower diameter and where the tensile strength is expressed in linear density (tenacity). There is not, in this patent, any mention to typical fiber properties, such as linear density (denier) and tenacity.
U.S. Pat. No. 5,246,657 teaches a process for obtaining ultra-high molecular weight polyethylene fibers from a mixture of two olefin resins, one of them being polyethylene and the other one, a copolymer of PE and PP, in the presence of a wax diluent to facilitate extrusion. The polyolefin mixture is extruded and stretched at a draw ratio of at least 10.
U.S. Pat. No. 5,176,862 owned by DSM, a company with expertise in PE fibers for offshore applications, describes cables where the polymer obtained by gel spinning has been stretched in order to improve the mechanical properties.
U.S. Pat. No. 6,183,834 also owned by DSM describes a series of parameters for the PE fibers in terms of tensile strength and denier number by filament. This company uses gel spinning only for polyethylene spinning.
In the article by Roerdink, E and van Dingenen, J.—“Past and Future of High Performance Fibres”, Polymer Fibres 2002, 10-12 Jul., 2002, The Manchester Conference Centre, UMIST, Manchester, UK FIG. 1 shows tenacity values for several materials, beginning with the textiles that reach values in the range of 5.0 gf/den, carpets and hoses with values up to 10, and materials of the class of ropes, composites, ballistic and aerospace industry materials in the range of 20 and up to 40 gf/den. However, according to the caption of this Figure, there is a gap in tenacity values in the range between 10 and 20 gf/den, such range being advantageously covered by the fiber materials of the invention, as will readily be seen hereinbelow.
Thus, on the one hand, the literature points out the polyethylene production technology yielding products of acceptable mechanical properties but which do not necessarily render these polyethylene products suitable for fiber manufacture and on the other hand, the polyethylene fiber production technology based on ultra-high molecular weight materials that can be extruded only when dissolved in organic solvents according to the gel spinning process.
Therefore, the technique still needs a process for obtaining fibers from high-modulus, extrudable polyethylene by extrusion and stretching in state-of-the-art equipment, where said polyethylene is obtained by an ethylene polymerization process where the control of the ethylene/hydrogen ratio during polymerization as well as process features such as the catalyst being supported on varying amounts of mixed alumina and magnesium chloride lead to polyethylene grades having such a molecular weight distribution Mw and melt index MI21 values typical of high-modulus materials that are at the same time extrudable into fibers in the absence of any added solvents, such fibers and process for obtaining same being described and claimed in the present invention.